Monolitically integrated generator of a plurality of voltage values

Abstract: The present invention relates to a monolithically integrable predetermined voltage generator (1) comprising a node (IN) held at a constant voltage (Vpp) and a plurality of circuit branches (2a, 2b, 2c and 2d) each of which incorporates at least one turn on control terminal (3a, 3b, 3c and 3d) and which are interconnected in such a manner that at least one of them leads to said node (IN). Across each of the branches (2a, 2b, 2c and 2d) a corresponding voltage drop is present when the branch is turned on and obtained from a certain number of active elements included in the branch. In accordance with a preferred embodiment, diode-connected MOS transistors are used as active elements. The generator also comprises an output terminal (OUT) associated with at least one of said branches at which is generated a predetermined voltage value. A control signal is applied to at least one turn on control terminal (3a, 3b, 3c and 3d) of said plurality of branches for turning on a predetermined subset of said plurality of branches and generating at the output terminal (OUT) a voltage lower than that at said node (IN). Each of the predetermined voltage values is a predetermined combination of the voltage drops on the turned on branches. In a preferred application, the generator of the present invention is used in a multilevel EEPROM memory circuit for generating a plurality of programming voltages for the memory cells.

Description: Field of application The present invention relates to a monolithically integrable generator for selectively generating, one of a plurality of predetermined voltage values, especially for multilevel memories. The present invention also relates to a method for generating one of a plurality of predetermined voltage values in a monolithically integrated electronic circuit. Specifically the present invention concerns a programming voltage generator for non-volatile memories and specifically for EEPROM memories with multiple logic levels. The present invention also concerns a memory circuit incorporating a memory matrix and programming voltage generators of the above type. The following description is given with reference to this field of application with the main purpose of simplifying discussion thereof. Known art As is well known, in integrated circuits it is often necessary to have available a plurality of predetermined and discrete voltage values, readily distinguishable and accurately controllable. The voltage value is supplied to a user circuit and serves either as reference or as power supply for part of the circuit itself. In particular in the present description reference is made to non-volatile memory circuits and specifically to multilevel memories requiring different programming voltages. As known, multilevel memories are of great interest in the new technologies because they allow reduction of the size of the memory matrix. Indeed, in a multilevel memory it is possible to store a certain number of different logical states (more than two) in each individual memory cell. It is worth recalling that a memory cell is formed by means of a floating-gate MOS transistor and for each memory cell the logical states are distinct from the amount of charge stored in the floating gate. Standard non-multilevel non-volatile memories are programmed in two logical states which permit to store one bit per cell. In the case of multilevel memories it is possible to distinguish more than two stored charge states and correspondingly it is also possible to program multiple logical states in each cell. For example if it is possible to distinguish four or sixteen states in a cell, in each memory cell are stored two or four bits in conformity with binary coding. Said logical states or levels correspond electrically to different threshold voltage values of the cell and therefore to different programming voltages which must be applied to terminals of a cell or to terminals common to a series (row/column) of cells. The terminals to which are applied the programming voltages for a cell are the control gate terminals and/or drain terminals, while for a row and/or column of cells they are respectively the "word line" and/or "bit line" terminals. To obtain multilevel programming it is necessary in any case to have available predetermined voltage values, distinguishable with sufficient accuracy. For a level to be distinct from the contiguous ones its range of variation must obviously be less than the separation between two contiguous levels. In this manner is avoided an overlapping of the levels and therefore an indetermination which would cause a resulting incorrect multilevel programming. Generation of the voltage values necessary for programming a cell is dependent on the type of memory cell. In any case in any memory circuits are available at least one supply voltage Vcc, derived from an external line, and a programming voltage Vpp for programming the cells. The procedures for application of the programming voltage to the cell terminals are different. In the case of EPROM memories the programming voltage is generated externally. In the case of EEPROM memories, which are the preferred subject matter of the present patent proposal, the programming voltage Vpp is usually generated internally in the integrated circuit. Said voltage is generated by means of a so-called voltage pump circuit, to the input of it the power supply voltage Vcc, typically 5V, is applied to produce at its output a higher voltage Vpp, typically variable in a range 5V to 20V. For multilevel EEPROM memories it is also necessary to have different voltage values Vppn actually applied to the cell for its programming. Incidentally, in the different applications programming can require either positive or negative voltage values. For generating said predetermined voltage values there have been already proposed different circuit solutions. The programming method for a multilevel EEPROM cell at the desired value is specifically dependent on the type of the cell. With reference to EEPROM memory cells of the FLOTOX type, a programming method is described e.g. in European patent application no. 93830172.8, filed 22 April 1993 by this applicant. In said application there is described a circuit structure in which a capacitor, interposed between a drain node of a cell (i.e. the corresponding bit line) and a bit line voltage generator, acts as a programming voltage generator. By keeping the control gate of the cell polarised at a pre-set voltage value, there is charge injection in the floating gate by tunnel effect until formation of the channel in the cell and then discharge of the capacitor to ground with immediate interruption of programming. In this case the control gate voltage must be regulated with great accuracy. A programming algorithm divided in several steps, preferably with a rough adjustment and a subsequent fine adjustment which is interrupted when the chosen level is reached, is illustrated for the same type of cell in US patent no. 4,890,259. A similar regulation method in successive steps, for EEPROM memory cells with two polysilicon levels, is proposed in European patent application no. 93830061.3, filed 19 February 1993 by this applicant. In this specific case a voltage at a node which absorbs current, during the writing step, is regulated with great accuracy. Generation of voltage values Vppn, in conformity with the known techniques above mentioned, has the drawback that rather complex and sophisticated circuits have to be used to ensure adequate precision in distinguishing the logical levels to be stored. This entails technological efforts for realizing the generator, and requires manufacture of circuit elements by means of specific techniques different from those for forming the memory matrix. In addition, voltage generators in accordance with the known art require waste of a large part of integrated circuit area because of the presence of a considerable number of components. Therefore the cost of the device containing a memory matrix of a given capacity increases. A further known integrated circuit including an internal voltage generator is disclosed for example in US Patent 5,153,452. The voltage generator has been developed to overcome some technological problems such as latch up and coupling of the generator with the internal circuit which it is connected to. The disclosed solution includes a bipolar transistor driven by at least one MOS transistor selectably enabled by an external control signal. The voltage value at the output of the generator depends on an external voltage value supplied to the MOS transistor enabled by a corresponding power source. The transistors are realized so that the current path from the external power source to the output of the generator includes a N-type region insulated from the P-type substrate so as to avoid the above referenced problems. However the use of a bipolar technology render the generator dependent on technological process parameter. Japanese patent application JP3022860 discloses a circuit generating a voltage value less than a predetermined voltage of a fixed quantity, for use in testing EEPROM memory devices. For this scope it provides for two branches connected in parallel between a boosting device fournishing a fixed voltage value and an output node. Depending on the operation phase of the memory, the output node is connected respectively and alternatively through one of the branches to said fixed voltage or to said inferior value. In both branches a select element consists of a MOS transistor. The voltage drop in the second branch is determined by the presence of a certain number of diode-connected MOS transistors connected in series in the branch. A drawback of this solution is the large area occupied. Furthermore this solution generates only two voltage values. The technical problem underlying the present invention is to conceive a monolithically integrable generator for the generation of one of a plurality of predetermined and discrete voltage values, whose circuit structure would be simple and easy to provide. In this manner it would be possible to lower the production costs of the device containing the generator. At the same time the generator which is the subject matter of the present invention must have structural and functional characteristics allowing good adjustment of the voltage levels generated thereby. A particular purpose is to make available a generator of a plurality of voltage levels for programming non-volatile memory cells. Preferably such a generator must be integrable in a memory circuit in a limited space of the semiconductor chip, and fabrication technologies not different from those used for the memory cells themselves must be used for its integration. Summary of the invention The technical problem underlying the present invention is to provide a generator of predetermined and discrete voltage values by employing active elements exhibiting a voltage drop accurately predeterminable from their construction parameters. Each of a plurality of the predetermined voltage values is a predetermined combination of the voltage drops obtained at the generator output from a certain number of these active elements. A voltage generator in accordance with the present invention consists of different circuit branches, each of them comprising a single MOS transistor acting as active semiconductor element. Each branch exhibits a pre-set voltage drop which, as already mentioned, can be regulated accurately. The branches are connected together and selectively activated to generate every one of the plurality of the predetermined voltage values as a combination of the individual voltage drops. It is possible to use simple MOS transistors because a diode-connected MOS transistor transfers to its own source a predeterminable voltage, equal to the voltage applied to the drain minus its threshold voltage, increased as a result of the well-known body effect. On the basis of this solution idea the technical problem is solved by a generator of the type set forth above and defined in the characterising part of claims 1 and following. The technical problem is also solved by a method for generating a plurality of predetermined voltage values internally to a monolithically integrated electronic circuit, as defined in the characterising part of claim 8. The characteristics and advantages of the generator in accordance with the present invention are set forth in the description of an embodiment thereof given below by way of non-limiting example with reference to the annexed drawings. Brief description of the drawings In the drawings: FIG. 1 shows a block diagram of a voltage generator provided in accordance with the present invention, FIG. 2 shows a preferred embodiment of a generator in accordance with the present invention, FIG. 3 shows a generator according to a conventional technique, and FIG. 4 shows schematically an example of application to a memory circuit of the generator in accordance with the present invention. Detailed description With reference to figure 1, reference number 1 indicates as a whole and schematically a voltage generator provided in accordance with the present invention. The generator 1 includes an input node IN and an output terminal OUT. At the input node is kept a constant voltage Vpp, e.g. a programming voltage for a memory cell matrix. At the node OUT the generator produces a plurality of N predetermined and discrete voltage values Vppn. For generating said voltage values Vppn, the generator in accordance with the present invention comprises a plurality of M circuit branches, labelled as 2a, 2b, 2c and 2d. The value M can optionally be equal to N. In FIG. 1 are shown four branches by way of example, in accordance with a preferred embodiment of the present invention. Preferably the number M of the branches is on the order of a few units or tens. Each of the branches incorporates its own turn-on control terminal (3a, 3b, 3c and 3d in FIGS. 1a and 1b) which allows activation of the same branch. Each of these branches is characterised by a predetermined voltage drop Vm at its ends. The amounts of the voltage drops are different from each other. Said branches are connected together in the input node and parallel between the output terminal. Operation of the generator of FIG. 1 is as follows. A control signal, generated outside the circuit, is applied to at least one of the turn-on control terminals 3a, 3b, 3c and 3d. The control signal is shown in FIG. 1 with the arrows EN1, EN2, EN3 and EN4. This signal can be generated for example by a logical circuit, not indicated in the figures because conventional. The function of the control signal is to enable the corresponding branch to conduct. When a branch is selected, a voltage drop Vm is present at its ends. Said voltage drop is produced, for each of the branches, by a single active semiconductor element incorporated in the branch and illustrated in FIGS. 2 and 3. The control signal turns on one or more predetermined subsets of the entire plurality of branches so that there is a continuous conduction path between the input node IN and the output terminal OUT. In this manner the voltage drop at the output terminal, compared with the voltage at the input node, is produced by combination of the voltage drops of the turned on branches included in the conduction path. In practice, each branch is functionally activated or disconnected. The control signal acts selectively on only the branch which must be turned on. Now with particular reference to a preferred embodiment shown in FIG. 2, the structure of the generator in accordance with the present invention is described in greater detail. In said FIG. are shown the same reference numbers and symbols as in FIG. 1 for elements having the same structure and operation as the parts indicated schematically in those figures. The generator illustrated in FIG. 2 comprises circuit branches 2a, 2b, 2c and 2d connected in parallel between the input node IN and the output terminal OUT, similarly to the schematic representation of FIG. 1. The number M of branches, in this specific case four, is equal to the number N of a plurality of predetermined voltage values generated by the present circuit (M=N). Each branch includes a single MOS transistor, T1, T2, T3 and T4 respectively. Each of these transistors Tn comprises its own control terminal (gate terminals) 3a, 3b, 3c and 3d to allow to turn on the transistor itself and to activate the corresponding branch. The transistors Tn also include a drain terminal connected to the input node IN of the generator and a source terminal connected to the output terminal OUT. In addition to performing the function of branch selection elements, the transistors constitute in this preferred embodiment the active elements across which is present the voltage drop Vn, characteristic of each branch as mentioned above. Said voltage drop for each transistor is determined by the threshold voltage value of the transistor, as will be better explained below. In addition, in this preferred embodiment the transistors necessarily have different voltage drops, so that can be generated at the output terminal of the generator a plurality of distinct voltage values. During operation of the circuit the control signal (ENn) is applied to only one of the gate terminals of the transistors Tn so as to turn on only that transistor. Specifically, the control signal ENn imposes at the control terminal of the chosen transistor a voltage equal to the voltage Vpp present at the input node of the generator. The transistor is in a "diode type" configuration, with gate and drain terminals at the same voltage. In this configuration there is a pre-set voltage drop between drain and source, equal to the threshold voltage Vth(Tn) of the MOS transistor and dependent on the chosen transistor. In this manner at the output terminal OUT is generated one of a plurality of predetermined voltage values Vn, less than the input voltage Vpp. The generator in accordance with the present invention in its preferred embodiment therefore generates at the output terminal OUT a plurality of predetermined and discrete voltage values equal to Vout=Vin-Vth(Tn), where Vout is the voltage at the output terminal OUT, Vin is the voltage at the input node IN, i.e. Vpp, and Vth(Tn) is the threshold voltage of the transistor Tn when the branch n is selected. As mentioned above, in this embodiment it is necessary that the transistors Tn have distinct threshold voltages Vth(Tn). It is worth to recall that, for integrating in a chip transistors having a predetermined threshold voltage, the semiconductor must be appropriately doped. Specifically, during the steps for forming the transistor, the surface of the semiconductor is masked and, subsequently, dopant elements are implanted on the uncovered zones. This necessitates to have available a threshold calibration system having a certain dose value to be implanted and/or a certain energy. In accordance with the present invention in which the different thresholds are N, it is not however necessary to execute N masking and implanting operations. To minimise the number of masks it is possible to obtain different dopings by means of different combinations of a smaller number of implantations. By way of example, in the specific case illustrated in FIG. 2 in which are necessary four distinct thresholds, the embodiment is especially simple from the technical viewpoint. In this case only two dose implantations, D1 and D2 (e.g. D2 = 2*D1), and thus only two masks, are needed to obtain all the transistors. In particular, T1 is always masked and therefore is not implanted and Vth(T1) = Vth, natural threshold of the transistor. For the other transistors one proceeds respectively with the following implantations: D1 for the transistor T2 (Vth(T2) = Vth + DVth1), D2 for T3 (Vth(T3) = Vth + DVth2) and D1 + D2 for T4 (Vth(T4) = Vth + Dvth1 + DVth2). In present applications this type of implantation is already present in the circuit manufacture technology. More specifically, in the preferred embodiment, the transistors Ti are formed by CMOS technology. In a standard CMOS process, e.g. in case of multilevel EEPROM non-volatile memory for forming the memory matrix, two implantations are already provided. Therefore the additional operations for forming the generator in accordance with the present invention constitute a limited complication in comparison with a process of formation of the device without generator. Specific use of CMOS technology also allows generation at the generator output of both positive and negative voltage values by using n-channel transistors or p-channel transistors respectively, and inverting in the latter case the polarity of all the voltage values. As an alternative, since the threshold voltage of a MOS transistor depends on the gate oxide thickness, there can be achieved transistors with different threshold by intervening on this parameter, optionally in conjunction with a multiple implantation as described above. It should be noted that the generator in accordance with the present embodiment of the invention is especially simple and compact in terms of area occupied. The circuit is also easy to be made with no, or minimal technological effort, especially if, as illustrated in FIG. 2, the number of voltage values to be generated is not too high. The embodiment of the present invention in accordance with the diagram of FIG. 2 is especially suitable in the latter case where a limited number of voltage values is desired. It is also noted that it is possible to regulate with great accuracy the voltage drop across each branch. Both implantation and oxidation process parameters can be readily controlled with a variability (the so-called process .spread) very small. The threshold voltage value can be predetermined by the construction parameters (doping and/or gate oxide thickness) and the spread is a known characteristic. On the other hand, stability is ensured by the considerable simplicity of the circuit. An indicative value of the difference in voltage between two contiguous levels obtainable by a generator in accordance with the present invention is around 500 ±150mV. Incidentally, it is worth to note that with the present embodiment it is possible to regulate a number of levels not necessarily an exact power of two. With reference to FIG. 3 will be now described a generator with branches in parallel not falling within the present invention. Details and co-operating parts having the same structure and operation as the above embodiment are given the same reference numbers and symbols. FIG. 3 shows a generator comprising M circuit branches 2a, 2b, 2c and 2d connected in parallel between the input node IN and the output terminal OUT, similarly to the case illustrated in FIG. 2. For comparison with the embodiment shown in FIG. 2, four of them are shown in this example also. The number M of the branches is equal to the number N of the plurality of predetermined voltage values generable at the output terminal OUT (M=N). Every n-th circuit branch includes a progressive number n of MOS transistors connected in series. In particular for each branch the n transistors comprise a first transistor (pass transistor) T1, T2, T3 and T4 connected through its own drain terminal to the input node IN and having a gate terminal (turn-on control terminals 3a, 3b, 3c and 3d) for controlling transistor turn-on. The transistors Tn act as selection transistors for turning on the corresponding branch to which they belong. The branches comprise also n-1 transistors Tni, where n=1,..N indicates the branch under consideration, i=1,..n-1 indicates the position of the transistor in the branch starting from the one nearest the selection transistor. The transistors Tni are diode-connected in series between a source terminal of the corresponding selection transistor Tn and the output terminal OUT. These transistors Tni, because of their diode configuration, as described above for the embodiment of FIG. 2, have the function of generating at their source terminal a certain voltage, equal to the voltage applied to their drain terminal minus the threshold voltage. Said transistors are preferably all equal and have in particular the same threshold. The total voltage drop Vn for each circuit branch therefore increases progressively. Again in this case, the operation takes place turning on by means of the control signal (ENn) only one of the transistors Tn. Specifically, the gate terminal of the chosen transistor Tn is polarised at a voltage less than or equal to the voltage present at the drain, i.e. at the input voltage Vin = Vpp. Assuming for example that the third branch is activated by polarising the gate of T3 at a value Von, at the output OUT of the generator is obtained a voltage Vout = Vin - Vth(T3) - Vth - Vth, where Vout is the voltage at the output terminal OUT, Vth(T3) is the threshold voltage of the transistor T3, and Vth is the threshold voltage, affected by the body effect, of the two diode-connected transistors. Furthermore, if a current absorption from the output terminal OUT is expected, it is necessary to remember that across the transistors is also present a voltage drop between drain and source depending on the current (point of work). To minimise this problem there is provided at the output an additional transistor, having its gate connected to the output terminal and drain held at the voltage value Vpp. The circuit described, while being equally simple to that shown in Figure 2 from the structural viewpoint, is less compact than that illustrated in FIG. 2. To produce the voltage drops in the embodiments of the present invention it should be recalled that the threshold of a transistor is not constant but increases with increasing of the voltage applied thereto, because of the well known body effect. Therefore the separation between the voltage values obtainable at the output is not constant. However this does not constitute a problem in an application in which it is necessary to be able to discriminate only between two contiguous levels. In some applications this phenomenon could be advantageous. If the difference in voltage between successive levels must be a constant quantity, it would be possible to have recourse to a differentiation of the transistors on each of the branches. It is also noted that the voltage Vpp applied to the input node IN in some applications can take on different values, e.g. values obtainable from different stages of a voltage booster circuit. FIG. 4 shows a preferred example of application for the generator of the present invention to a memory circuit. In the figure, reference number 5 indicates a memory cell matrix, especially a multilevel EEPROM memory. The matrix 5 is conventional, and organised in M+1 rows and N+1 columns defined by conductive metallization lines called respectively word lines (WLO,...WLM) and bit lines (BLO,...BLN). At each intersection of a row and a column is placed a memory cell 6, formed by means of a floating-gate MOS transistor having its drain connected to the corresponding bit line (BLO) and its control gate to the corresponding word line (WLM). For multilevel programming of each cell (write or erase) to its gate and/or drain terminals there must be applied one of a plurality of predetermined voltage values. Therefore said terminals are connected alternately to a generator 1 by means of selection circuitry 7, e.g. switches, which permit selection or rather addressing of the chosen cell. The generator 1 is provided in accordance with the present invention, and as described above. The input node of the generator in this application is connected to the voltage Vpp, the maximum programming voltage generated for example by a voltage pump circuit. One of a plurality of predetermined voltage values, lower in absolute value than the voltage Vpp, is generated at the output terminal OUT of the generator in the manner set forth above. The memory cells are programmed by conventional methods. The generator in accordance with the present invention thus permits generation of a plurality of predetermined and discrete voltage values by employing active elements exhibiting a voltage drop which can be predetermined by the construction parameters. Preferably has been described the case in which diode-connected MOS transistors are used as active elements. Furthermore, the turning on each branch, which in the embodiments described is achieved by means of a MOS transistor, could take place by means of any switch. Of course modifications and variations can be made to the generator described above by way of non-limiting example all however falling within the protective scope of the following claims.

Claims: Monolithically integratable generator (1) for selectively generating one of a plurality of predetermined voltage values, the generator (1) having at least one node (IN) held at a constant voltage (Vpp) provided externally to the generator (1), the generator (1) receiving at least one control signal (EN1, EN2, EN3, EN4) at at least one of a plurality of turn-on control terminals (3a, 3b, 3c, 3d) internal to the generator (1) in order to provide said one of said plurality of predetermined voltage values at an output terminal (OUT) of the generator, wherein the generator (1) comprises a plurality of circuit branches (2a, 2b, 2c, 2d) connected together in parallel between the node (IN) held at said constant external voltage (Vpp) and the output terminal (OUT), each branch comprising at least one active element providing a corresponding voltage drop for each branch, said corresponding voltage drops being different in each branch, each branch incorporating at least one of said at least one of a plurality of turn-on control terminals (3a, 3b, 3c, 3d), and one of said at least one control signals (EN1, EN2, EN3, EN4) when applied to at least one of said plurality of turn-on control terminals (3a, 3b, 3c, 3d) of said plurality of branches turns on the associated one of said plurality of branches and generates at the output terminal (OUT) a total voltage drop with respect to said node (IN) held at said constant external voltage (Vpp), the generator being characterized in that each of said branches comprises a single MOS transistor acting as active element and for turn on control of the respective branch, having a gate terminal connected to said turn on control terminal. Generator according to claim 1, characterised in that for each branch the corresponding MOS transistor is diode-connected. Generator according to claim 1, characterised in that the MOS transistor of each branch has a drain terminal connected to said node, and a source terminal connected to said output terminal. Generator according to claim 3 characterised in that said turn on control signal for one of said branches applies to the gate terminal of the respective MOS transistor a polarisation voltage having a value equal to said constant voltage. Generator according to claim 1, characterised in that it is formed by means of CMOS technology. Memory circuit comprising: a memory cell matrix of the multilevel EEPROM type organised in rows and columns and having drain and control gate terminals connected respectively to the columns and rows of the matrix, selection circuitry connected to each end of the columns and rows to select the column or row chosen during the programming step, and at least one generator according to one of claims 1 to 5. Memory circuit according to claim 6, characterised in that it is completely formed by means of CMOS technology. Method for selectively generating one of a plurality of predetermined voltage values at the output terminal of an electronic circuit, comprising the following steps: providing for a plurality of circuit branches of said electronic circuit, each of said plurality of circuit branches being interlocked with an activation signal and containing a single MOS transistor acting as active element and for turn on control of the respective branch, arranging an interconnection network between said branches, including a parallel arrangement of said circuit branches, selecting by means of the activation signal one of said plurality of circuit branches to obtain a corresponding voltage drop present across said active element of the selected circuit branch, the corresponding voltage drops being different for each branch, said selecting step being performed so as to activate a selectable conduction path between a terminal held at a constant voltage and said output terminal, and to obtain said predetermined voltage value from the voltage drop through the selected branch.

Inventors: PARUZZI PAOLA, PIO FREDERICO, VAJANA BRUNO, PARUZZI, PAOLA, PIO, FREDERICO, VAJANA, BRUNO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1994
Main classification: G11C11
IPC codes: G11C16, G11C11, G05F3, G11C17, G11C5